Microwave harmonic generator



Nov. 22, 1960 E. STERN mcRowAvE HARMONIC GENERATOR Filed June 2, 1958 www 35 ggf. INVENTOR ERNEST TER/v I\ Ill.

ATTORNEY States Patent- 2,961,615 l MICROWAVE HARMoNIC GENERATOR Ernest Stern, New York, N.Y., assignor to Sperry Rand Corporation, Great Neck, N.Y., a corporation of Delaware Filed June 2, 195s, ser. No. 739,272

9 claims. (cl. ssl-77) This invention relates to microwave generators and more particularly to passive microwave harmonic generators employing ferromagnetic material.

There is presently a demand for microwave generators which are capable of producing energy at useful power levels in the upper microwave frequency bands. Electromagnetic energy in the millimeter wave region of the frequency spectrum has been generated in the past by means of harmonic generators which employ crystals having a very fine wire in contact with the surface of a semi-conductor material. Harmonic generation results from the non-linear barrier behavior of the semi-conducltor material when an electrical voltage Vis impressed thereon. These crystal harmonic generators Vsuffer from the defects of being mechanically and electrically un-y stable and are easily destroyed when subjected to shock. Additionally, the crystals cannot operate at high power levels and will burn out at an instantaneous power level of approximately one watt. Consequently the generated second harmonic energy is at a relatively low power level.

It is therefore an object of this invention to provide a microwave harmonic generator which is capable of operating at high power levels.

It is another object of this invention to provide a microwave harmonic generator which is both mechanically and electrically stable.

It is a further object of this invention to provide a microwave harmonic generator which is capable of generating appreciable amounts of millimeter wave energy.

These and other objects which will become more apparent as the description proceeds are achieved by providing a section of rectangular waveguide having a member of ferromagnetic material positioned therein. A microwave energy source supplies linearly polarized energy at a fundamental microwave frequency to the waveguide section. The ferromagnetic material is immersed in a unidirectional magnetic field which is directedgparrallel to the longitudinal axis of the waveguide and when the strength of the unidirectional magnetic field is sufficient -to magnetically bias the ferromagnetic material to the vicinity of the gyromagnetic resonance condition, and when the microwave energy is at a relatively high power level, appreciable amounts of second harmonic energy will be generated by the ferromagnetic material.

The magnetic field of the generated second harmonic energy will have a component parallel to the direction of the applied unidirectional magnetic field, and the second harmonic energy will propagate in the waveguide section. Means are provided for selectively propagating the second harmonic energy to a utilization means and for rejecting the energy at the fundamental frequency.

The invention will be described in connection with the following drawings wherein:

Fig. 1 is a vector diagram used to explain the phenomenon of harmonic generation in a ferromagnetic maten'al;

rf' ,961,614 CC 2 7 Fig. 2 is a plan view illustrating the novel harm ,mei

generator of this invention;

Fig. 3 is a series of sectional views taken at diffrent Ferromagnetism in ferromagnetic materials results from',`

the magnetic fields associated with spinning atomic elec?V trons in the material, wherein the electrons behave in a gyroscopic manner with their magnetic moments lying along their spin axes. When the solid ferromagnetic material is magnetized to saturation by a unidirectional applied magnetic field and a component of the microwave magnetic field is applied to the material in a direction transverse to the unidirectional magnetic field, the magnetic moments of the electrons precess about the direction of the applied unidirectional magnetic field.

The interaction between the microwave field and the ferromagnetic material may be described by the vectorequation `Y =w ,fa-MT 9 where M is the resultant magnetization vector Vof the ferromagnetic material; l' is the total applied magnetic; field which is composed of the microwave magnetic field` and the unidirectional applied magnetic field; fy is the gyroma'gnetic ratio of an electron in the ferromagnetic material; and T is the relaxation time of the electron precession.

In the early analyses of this equation the microwaveA component of magnetization along the direction .of thek unidirectional magnetic field had been neglected because: at small microwave signal levels that component is extremely small. However, if this component is not neglected and is further analyzed, it may be shown that if lthe microwave field is a complex function of time, i.e., ewt, the component of magnetization parallel to the direction of the applied unidirectional field contains a secondv harmonic component. v

The following simplified analysis will reveal the ex-,lV istence of the second harmonic component.

Equation 1 may be expressed in component form as 952-4 M H :@[tMyHf MJL) -Mfl netization vector M may also be represented in component forms as Y l and . Patented Nov.

Y where Ho is the unidirectional or steady magnetic field ,i Y

Substituting the components of Equations 3 and 4 for the corresponding components in Equation 2 and equating the components of Equations 2 and 5 gives the following expressions:

's tmawamanamak (5) Because the linearly polarized microwave magnetic eld is a complex function of time, the component of that field may `be written in the following form:

In order for the equality of Equations 6 and 7 to hold true when Equations 9 and 10 are substituted therein, the following must also be true:

Considering now Equation 8, the expression for the component of magnetization in the direction of the unidirectional magnetic field, and substituting Equations 9-12 into Equation 8 results in the expression:

om, Mz -t= ezlwn'xh'y-m'yhQ-f-TT (13) From Equation 13 it may be seen that so long as m'hy is not equal to myhX (the microwave magnetic field is not`circularly polarized) a second harmonic component of the magnetization exists in the direction of the applied unidirectional magnetic field,

By solving for mx and my in Equations 6 and 7 and substituting the solutions into Equation 13, it is also revealed that the expression contains a term (hx2+h,2)

indicating that the second harmonic power level should vary as the square of the fundamental microwave fre quency power level. a

The concept of second harmonic generation may be illustrated graphically as in Fig. l where the vector l represents the magnetization vector of the ferromagnetic material that precesses at the fundamental microwave frequency about the Z axis, which is the direction along which the unidirectional magneticfeld Ho is applied. The path followed by the tip of the precessing vector is shown by a dotted line. The projection of this path on the X-Y plane is an ellipse. Since the magnitude of the vector is constant, the tip of the vector rises and falls twice each time it transverses the elliptical path, and a second harmonic component of magnetization is generated along the Z axis.

The angle of precession of the vector is greatest at gyromagnetic resonance, so it may be seen from Fig. l that the generated second harmonic should be greatest at gyromagnetic resonance. At low microwave power levels, Vthe precession angle of vector about the Z axis opens up to a steady state value as the gyromagnetic reso nance condition is reached. As the power is increased, the precession angle opens up proportionally until it reaches a critical value, after which an additional increase in power will produce a lesser non-linear increase in the precession angle. Therefore, the harmonic generator of this invention should be operated at relatively high micro- Wave power levels. The limit of the high power level will be determined by the heating of the ferrite by the incident microwave energy, and by certain non-linear effects which will set in at high power levels. As of the present time, the non-linear effects have not been fully investigated.

Although it would intuitively appear that the greatest amount of harmonic energy would be generated when the ferromagnetic material is magnetically biased exactly to gyromagnetic resonance, I have found that in an operable device constructed according to this invention, optimum second harmonic energy was generated when the ferromagnetic material was biased very slightly away from the exact gyromagnetic resonance condition. A possible explanation for this phenomenon is that ferromagnetic material absorbs the incident fundamental energy very rapidly when biased exactly to gyromagnetic resonance and only a small portion of the material is subjected to the microwave field. Thus, if the ferromagnetic material is magnetically biased slightly away from the exact gyromagnetic resonance condition the microwave magnetic field will not be attenuated as rapidly by the material and optimum harmonic generation is obtained.

One embodiment of the novel harmonic generator of my invention is illustrated in Fig. 2 which is a plan view showing a source 11 which supplies microwave energy at a fundamental frequency f1 at a `relatively high power level to a section of rectangular waveguide 12 which is in turn coupled to waveguide' section 13. The dimensions of waveguide section 13 are chosen so that microwave energy at frequency f1 can propagate only in the dominant TEM, waveguide mode, and energy at the second harmonic frequency 2f1 can propagate in a transverse electric mode no higher than the TEZO mode. Disposed within waveguide section 13 are two ferrite elements 14 and 15' which are disposed along the longitudinal axis of the waveguide, Fig. 3b.

The ferromagnetic material comprising ferrite members 14 and 15 is preferably la material having a low dielectric loss. A more vigorous analysis of the equations presented above has revealed that for optimum second harmonic generation the ferromagnetic material should have a narrow linewidth and a high value of saturation magnetization Ms. I have found that Ferramic R-l, a polycrystalline ferrite produced by General Ceramics Corporation, and a Linde single crystal Mg n ferrite, produced by LindeAir Products Company, may be used successfully tongenerate second harmonic energy. The ferromagnetic elements may take the shape of thin slabs as illustrated in Fig. 2, or they may take other forms, such as cylinders or rectangular prisms for example, whose center axes are normal to the broad walls of the rectangular waveguide section 13. The dimensions of the ferrite members should be suiciently small so that the desired propagating modes in the waveguide are not seriously perturbed.

Positioned at the end of waveguide section 13, and also disposed along the longitudinal axis, is .a non-reflecting microwave attenuating card 16. Disposed about waveguide section 13 is an'electromagnet coil 1'7 which produces a unidirectional magnetic field Ho which is directed along the -longitudinal axis of waveguide 13 in the region occupied by ferrite elements 14 and 15. Magnetic field Ho is `of the correct strength to bias ferrite elements 14 and 15 tothe Vvicinity ofthe gyromagnetic resonance condition.

"Coupled to waveguide section 13 is a waveguide section 18 which has a conductive septum 19 positioned along theplongitudinal axis thereof, Fig. 3c. Conductive septum 19 extends between the broad walls of waveguide section 18 and divides it into two substantially equalf enemies waveguide sections 20 and 21 whose broaddimensions are approximately one-half as wide as the broad dimensions of waveguides 12, 13, and 18. Each of the smaller sized waveguides 20 and 21 are beyond cut-olf to energy at the fundamental frequency f1, consequently the fundamental frequency energy fl cannot propagate therein.,

The dimensions of waveguides 20 and 21 are such that energy at the secon-d harmonic frequency 2]1 can propagate in each waveguide in the TEM, modes only. A coupling aperture 22 is located in conductive septum 19 and comprises a 3 db directional coupling means which electrically couples the smaller waveguide sections Ztl and 21. Coupled to the ends of waveguide sections 2t) and 21 are waveguide sections 23 and 24, respectively. A non-reflecting energy absorbing load 25 terminates waveguide section 24, and waveguide section 23 is coupled to a utilization device 26.

A microwave phase shifter 27 is positioned in the section 'of smaller sized waveguide 21 and is adapted to introduce a 90 phase delay to microwave energy at the second harmonic frequency 2h which propagates through waveguide section 21.

In the operation of the harmonic generator of Fig. 2, linearly polarized microwave energy at frequency f1, at arelatively high power level, propagates in the TEN mode from source 11 into waveguide section 13, Fig. 3a, and encounters ferrite elements 14 and 15 which are immersed in the axially directed unidirectional magnetic field H0. The strength of magnetic eld HD is sufficient to magnetize ferrite elements 14 and 15 approximately to gyromagnetic resonance at frequency f1.

The relationship between the microwave magnetic field and the steady magnetization of ferrite elements 14 and 15 may be seen by referring to the illustration of Fig. 4. The microwave magnetic eld o-f a wave propagating in the TElO mode is represented` in the form of a closed loop whose plane is parallel to the broad walls of the waveguide. The microwave field is comprised of a component hz which is directed parallel to the axis of the waveguide, and a component hx which is directed normal to the axis of the waveguide. It may be seen that the hx component is perpendicular to the unidirectional magnetic iield H0, and because the ferrite elements are magnetized approximately to gy-romagnetic resonance by the unidirectional iield Ho, the conditions necessary for second harmonic generation, as specified above, are present.

Therefore, second harmonic energy at frequency 2h will be generated by the ferrite elements and will propagate in waveguide section 13. The mathematical development above indicated that a component of the magnetic eld of the generated second harmonic energy will be in the Z direction, i.e., the direction parallel to the unidirectional magnetic field H0. The generated second harmonic magnetic field can couple to any waveguide mode that has a component of microwave magnetic field in the Z direction in the region occupied by the ferromagnetic material. By accurately placing ferrite elements 14 and 15 in the center of waveguide 13, and by making the ferrite elements of such size and shape that they do not introduce serious perturbations to the waves propagating in the waveguide, a major portion of the second harmonic energy will propagate in the asymmetrical TE20 mode, Fig. 3b. To further assure that the second harmonic energy will propagate predominantly in the TEE@ mode and will not pro-pagate in a transverse magnetic mode, the height of waveguide section 13 is approximately `one-half the height of a normal waveguide having the same broad dimension as waveguide section 13. That is, the broad dimension of a waveguide normally falls between 1g and g/z, where kg is the wavelength within the waveguide, and the narrow dimension is normally made equal to or less than )rg/2. Therefore, in practicing the present invention the narrow dimension of the waveguide section 13 is equal to or less than am.

`The second harmonic'energy will tend to propagate; in both the forward and backward directions from ferrite elements 14 and 15. However, by spacing the ferrite members apart a distance d which is substantially equal to an odd multiple of a quarter waveguide wavelength at the second harmonic frequency propagating in the TEZQ mode, the second harmonic energy generated by ferrite elements 14 and 15 will substantially cancel in the direction toward source 11, and will constructively add in the forward direction and will propagate toward waveguide 18 and utilization means 26. As an example, assume that the generated second harmonic energy is propagating in waveguide 13 in the TEZO mode and that the spacing d between ferrite elements 14 and 15 is one-quarter waveguide wavelength at the second harmonic frequency 2h, and consider that for reference purposes all phase measurements as being made from ferrite element 14. Second harmonic energy generated by ferrite 14 and propagating to the right in waveguide 13 in the TE20 mode will arrive at ferrite 15 with a phase delay of 90, having propagated a distance d equal to a quarter waveguide wavelength. Microwave energy at the fundamental frequency f1 in the 'fl-310 mode which induces second h-armonic generation at ferrite 15 will arrive at ferrite 15 with a phase delay of 45 since its waveguide wavelength is twice that of the second harmonic propagating in the TEZO mode. Because the generated second harmonic power is proportional to the square of the fundamental power, the second harmonic energy generated by ferrite 15 will haveV twice the phase angle of the exciting fundamental energy, that is, a phase angle of Therefore, the second harmonic energy propagating to the right from ferrite 14 will combine in phase with the second harmonic generated by ferrite 15 and they will constructively add and propagate to the right.

Second harmonic energy generated by ferrite 14 and propagating to the left in waveguide 13 will have a zero phase shift, since it is at the chosen phase reference position. The second harmonic energy generated at a phase angle of 90 by ferrite 15 will be delayed 90 in propagating to the left the distance d, and will arrive at ferrite 14 with a total phase displacement of 180 with respect to the second harmonic energy generated by ferrite 14. The two components will therefore combine in. destructive interference and will substantially cancel with the result that substantially no second harmonic energy will propagate tothe left in waveguide 13.

The microwave energy at the fundamental frequency fywill be ,partially absorbed by ferrite elements 14and '15 since they are operating in the vicinity of gyrornagnetic resonance at frequency f1, and will also be ab- Y If all of the energy at Y the fundamental frequency is not absorbed, it will be presorbed by attenuating card .16.

field of the asymmetrical TEZO mode is substantially zero in the center of the waveguide 13. The second harmonic energy vpropagating in the asymmetrical TEz mode will enter waveguide section 18, and because conductive septum 19 comprises a common narrow wall between smaller sized waveguides 2t) and 21, the two halves of the TEZU mode will propagate independently as two TEM modes in waveguides 2i) and 21, respectively. For reference purposes, it will be assumed that the energy propagating in waveguide 21 lags in phase'by 180 the energy prop-v` agating in waveguide 2t), Fig. 3c. Energy in Waveguide 2@ will continue to the right and will encounter 3 db directional coupling aperture 22, and energy in waveguide 21 will continue to the right and will experience a 90 phase delay in propagating past phase delay means 27, and will arrive atv coupling aperture 22 with a total phase displacement of 270 with respect to the energy in waveguide 20.

It is well known to those skilled in the art that when energy is incident on a 3 db directional coupler from one waveguide, the coupler functions to divide the incident energy substantially equally between the two waveguides on the far side of the coupler, and it is also known that a 90 phase difference is introduced between the two equal components. In the discussion which follows, the component of the incident energy which is coupled into the opposite waveguide will be referred to as the coupled component and the component which propagates past the coupler in the same waveguide as the incident energy will be referred to as the direct component. The phase shifts referred to below represent only the relative phase differences between components.

The coupled component of the energy incident on the coupler from waveguide 20 will experience phase delay of 90 in passing through the coupler and will combine in waveguide 24 with the direct component of the energy incident from waveguide 21 which has a phase displacement of 270. Because these two components are 180 out of phase, they will combine in destructive interference and substantially no energy will propagate in waveguide 24. Lossy termination 25 will absorb any energy which may propagate in waveguide 24 as a result of incomplete cancellation of the two components. The energy incident on 3 db coupler 22 from waveguide 21 will be divided by coupler 22 into two substantially equal components which will tend to enter into waveguides 23 and 24. The coupled component will experience a 90 phase delay in passing through the coupler and will have a total phase displacement of 360 with respect to the direct component of energy incident from waveguide 20. Therefore, these two components will add in phase in waveguide 23 and substantially all the energy will propagate toward utilization device 26.

Therefore, by establishing the correct phase relationship between the components of the energy propagating in waveguide sections 20 and 21 substantially all the generated second harmonic energy will propagate in waveguide 23 to utilization device 26.

It is to be understood that the present embodiment of the invention may be practiced by employing just one ferrite element in waveguide section 13. It has been demonstrated, however, that under such circumstances approximately one-half of the second harmonic energy will not propagate to utilization device 26.

Fig. is a diagrammatic showing of another embodiment of the present invention wherein the section of waveguide containing the ferromagnetic `material is placed in a waveguide loop 32 of the type disclosed in patent application SN. 482,076, filed January 17, 1955, now Patent No. 2,875,415, in the name of Peter I. Sferrazza, and assigned to applicants assignee. Ferromagnetic member 30 is centrally positioned on a broad wall of rectangular waveguide section 31, which comprises a part of the closed waveguide loop 32. Waveguide section 31 is similar to waveguide section 13 of Fig. 2 with the exception that an attenuating card is not included in waveguide section 31. The electrical length of waveguide loop 32 is substantially equal to an integral multiple of a waveguide wavelength at the frequency f1. A source of microwave energy 33 at frequency f1 is coupled to one end of waveguide section 34, and a nonreecting termination 35 is coupled to the opposite end of waveguide 34. Directional coupling means, indicated generally at 36 couples waveguide section 34 to closed loop 32. Coupling means 36 may be either a xed coupling means or a variable coupling means. Directional couplers 37 and 3S are positioned at the ends Of waveguide sections 45 and 46 and each are adapted to couple only microwave energy at the second harmonic frequency from the` waveguide loop into waveguide sections 39 and 40, respectively. Directional couplers 37 and 33 may each take the form of an array of coupling apertures which are substantially centered on a broad wall of the respective waveguide sections in waveguide loop 32. Since the coupling apertures are substantially centered on the broad walls of the waveguides, they will have very little affect on the energy at frequency f1 propagating in the TEM, mode in the waveguide loop. However, the magnetic field of the harmonic energy propagating in the TE20 4mode will be maximum at the center of the waveguide and the centrally positioned coupling apertures will couple the harmonic energy into waveguides 39 and 40. Waveguide sections 39 and 40 are beyond cut-off to microwave energy at frequency f1 and have dimensions such that energy at the second harmonic frequency 2f1 will propagate in them only in the fr E10 mode.

An electromagnet ift2, for example, comprises means for immersing ferromagnetic member 30 in a unidirectional imagnetic eld Ho which is directed along the longitudinal axis of waveguide section 31. Electromagnet 42 provides a eld strength of the correct value to magnetically bias rferromagnetic member 30 approximately to gyromagnetic resonance. The waveguide section 31 andv the waveguide sections 45 and `46 have broad and narrow walls whose dimensions are proportioned in the same manner as the corresponding walls of waveguide section 13 of Fig. 2 in order that the generated second harmonic enel-g3,r will propagate predominantly in the T1520 mode in those waveguide sections.

1n the operation of the device diagrammatically illustrated in Fig. 5, microwave energy at a rst frequency f1 from source 33 propagates within waveguide section 34 toward directional coupler 36. A portion of this energy is coupled into closed loop 32 and propagates therein in the form of traveling waves. When the electrical length of closed loop 32, the coupling value of coupling means 35, and the attenuation in the closed loop are correctly proportioned, as taught in the above-mentionedA application by Sferrazza, the power level in the loop may be made to exceed the power level of the source 33, and because the power level of the generated second harmonic energy is proportional to the square of the microwave energy at frequency f1 incident on the ferromagnetic material, the power level of the generated second harmonic energy is increased without the need for a higher power generating means at source 33.

Because ferromagnetic member 30 is magnetically biased to the vicinity of gyromagnetic resonance at frequency f1 it will introduce attenuation in the loop 32. As mentioned previously, this attenuation must be proportioned with respect to the coupling value of directional coupler 36 and the length of the loop 32 so that power multiplication is in fact achieved within the loop 32. An acceptable value of attenuation in loop 32 may be achieved by the proper choice of the size and type of ferromagnetic material employed, as well as by the strength of the magnetizing eld. As previously discussed, the ferromagnetic material may be magnetically biased slightly away from the exact gyromagnetc resonance condition in order to reduce the attenuation of the energy at the fundamental frequency f1. These factors will also affect the amount of harmonic energy generated by the ferromagnetic member 30, and in many instances a compromise will have to be arrived at in order to 0btain optimum harmonic generation.

In a manner similar to that described above, ferromagnetic member 30 will generate second harmonic energy which will propagate in the TE20 mode in both directions in waveguide section 31. The harmonic energy propagating in opposite directions will encounter the respective directional couplers 37 and 38 and will be coupled into waveguide sections 39 and 40, respectively. Microwave energy at frequency f1 will be prevented from propagating in Awaveguide, Sections 39 and 40 because both `of eener? 9 said waveguide sections are beyond cut-off toenergy at frequency f1, and by the fact that directional coupling means 37 and 38 are selective only to the harmonic energy.

The second harmonic energy propagating in waveguides 39 and 40 in TEN modes is coupled into a single waveguide section 41 by means of 3 db coupling means 43 in a manner similar to that illustrated in Fig, 2. Phase shifting means 44 is inserted in waveguide section 39 to establish the correct phase relationships between the energy propagating in waveguides 39 and 40 so that substantially all the second harmonic energy will couple into waveguide section 41 and will propagate to a utilization means not shown. Assuming that in the absence of phase shifting means 44 the two propagating paths of the second harmonic energy are substantially equal in length, wherein said two paths include waveguide sections 39 and 40, respectively, a phase delay of 90 introduced by phase shifting means 44 will cause substantially all the generated second harmonic energy to couple into waveguide section 41 and to a utilization device.

While the invention h-as been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departing from the true scope and yspirit of the invention in its broader aspects.

What is claimed is:

l. A microwave harmonic generator comprising a first section of hollow rectangular waveguide, means for coupling energy at a first microwave frequency into said waveguide section, said waveguide having dimensions such that microwave energy at the frequency of said source propagates therein in the TEN, mode only and microwave energy at twice the frequency of said first frequency cannot propagate therein in any transverse electric waveguide mode higher than the TEZO mode, a member of ferrite material positioned in said waveguide, and means for mmersing said member in a unidirectional magnetic field directed along the longitudinal axis of said waveguide section, said magnetic field being of the correct strength to magnetically b-ias said ferrite member substantially to gyromagnetic resonance, said ferrite member being adapted to generate microwave energy at a frequency which is harmonically related to said first frequency when immersed in said magnetic field and when microwave energy at said first frequency is incident thereon, means comprising two waveguide paths for coupling said generated harmonic energy from said first section of rectangular waveguide, said two waveguide paths being beyond cut-off to microwave energy at said first frequency and being adapted to propagate microwave energy at said harmonic frequency only in a TEN mode wherein the electric field vectors are parallel to the electric field vectors of the microwave energy at said first frequency propagating in the TElo mode in the first section of rectangular waveguide, and means for coupling said harmonic microwave energy from said two waveguide paths into a single waveguide path.

2, A microwave harmonic generator comprising a first section of hollow rectangular waveguide, means for coupling electromagetic energy at a first microwave frequency propagating in the TEN mode into said waveguide section, a member of ferromagnetic material substantially centered along one broad wall of said waveguide and extending parallel to the llongitudinal axis of said waveguide, said member being magnetically biased in a direction parallel to said axis to the approximate condition of gyromagnetic resonance at said first frequency, said magnetized ferromagnetic member being adapted to generate microwave energy at a harmonic frequency of said first frequency when said microwave energy at the first frequency is incident thereon, said rectangular waveguide section having `dimensions such that microwave energy at t 10 s said harmonic frequency verse electric mode 'polarized orthogonal to the polarization of the first frequency energy propagating in said waveguide but will propagate in said rectangular waveguide in a transverse electric mode no higher than the TE20 mode, wherein the electric field vectors of said' energy in the TEZO mode are parallel to the electric fieldr vectors of said energy at the first frequency propagating in said waveguide in the TEM) mode, means including two waveguide paths beyond cut-off to energy at said first frequency for coupling said generated harmonic energy from said first section of rectangular waveguide, and means for coupling the harmonic energy propagating in said two waveguide paths into a single waveguide path.

3. The combination as claimed in claim 1 wherein said first section of rectangular waveguide is in a closed waveguide loop which is substantially an integral number of waveguide wavelengths long at said first frequency, and wherein said two waveguide paths are disposed on opposite ends of said first section of rectangular waveguide, and including directional coupling means for coupling said source to said loop, and two coupling means responsive only to said harmonic energy for coupling said two waveguide paths to sa-id first section of rectangular waveguide.

4. A microwave generator comprising a source for supplying electromagnetic energy at a first microwave frequency, a first section of rectangular waveguide coupled to said source and being adapted to propagate microwave energy only in the TEM, mode at said first frequency and adapted to propagate microwave energy in a mode no higher than the TE20 mode at the second harmonic frequency of said first frequency, a ferrite member disposed in said waveguide section in a region where the magnetic field of said microwave energy is substantially linearly polarized, said ferrite member having a crosssectional area considerably less than the cross-sectional area of said waveguide, means for providing a unidirectional magnetic field directed' parallel to the longitudinal axis of said waveguide in the region occupied by said ferromagnetic member, said magnetic field being of sufiicient strength to magnetically bias said member substantially to gyromagnetic resonance, means positioned in said waveguide beyond the end of said ferrite member opposite said source for dividing said wavegu-ide into two substantially equal parallel waveguides having a common narrow wall therebetween, directional coupling means in said common wall for electrically coupling said two parallel waveguides, and phase shifting means disposed in one of said two parallel waveguides in a region between said ferrite member and said coupling means.

5. A microwave harmonic generator comprising a section of rectangular waveguide, a ferrite member disposed within said waveguide section, a microwave generating source for supplying microwave energy to said waveguide section, means for magnetically biasing said ferrite member in a direction parallel to the longitudinal axis of said waveguide to the approximate gyromagnetic resonance condition, said ferrite member being adapted to generate microwave energy at a harmonic frequency of said first frequency when immersed in said magnetic field and when microwave energy at said first frequency is incident thereon, conductive means disposed within said waveguide on the side of said ferrite member opposite said source and extending between the broad walls of said waveguide section for dividing said waveguide into two substantially equal smaller sized waveguides, wherein said conductive means comprises a common narrow wall between said two smaller waveguides, phase shifting means disposed within one of said smaller sized waveguides, anda directional coupling means located in said common narrowV wall, said phase shifting means being located betweenV said ferrite member and said directional coupling means.

6. A microwave harmonic generator comprising a first section of hollow rectangular waveguide, a member of ferromagnetic material disposed in said waveguide, a

will not propagaterin a trans-Ai 11- source of electromagnetic energy at a first microwave frequency coupled to said waveguide section, said rectangular waveguide section having dimensions such that microwave energy at the frequency of said source propagates therein in the TEN, mode only and microwave energy at twice the frequency of said first frequency propagates therein in a transverse electric mode no higher than the TEZD mode and will not propagate therein in a transverse electric mode polarized orthogonal to the polarization of energy at said rst frequency, means for immersing said ferromagnetic member in a unidirectional magnetic field which is directed along the longitudinal axis of said rectangular waveguide section, said magnetic field being of the correct strength to magnetically bias said member substantially to gyromagnetic resonance,

said ferromagnetic material being adapted to generate microwave energy at a frequency which is harmonically related to said first frequency when immersed in said magnetic field and when said microwave energy at the first frequency is incident thereon, said ferromagnetic member being substantially centered on a broad wall of said waveguide section and being narrow in relation to the broad dimension of said waveguide whereby said harmonic energy propagates in the waveguide predominantly in a T1520 mode whose electric vectors are parallel to the electric vectors of said energy at the first frequency propagating in the TEN mode, means comprising second and third waveguide paths for coupling said harmonic energy from said first rectangular waveguide section, said second and third waveguide paths being beyond cut-off to microwave energy at said first frequency, and means for electrically coupling said two waveguide paths to a single waveguide path.

7. The combination of claim 6 wherein said two waveguide paths are parallel extending rectangular waveguide sections having a common narrow wall therebetween, and wherein the broad walls and uncommon narrow walls of the respective parallel waveguide sections lie in planes respectively parallel and contiguous to the broad and narrow walls of said first section of rectangular waveguide.

8. The combination as claimed in claim 7 wherein said member of ferromagnetic material is comprised of two elements which are longitudinally spaced along said first rectangular waveguide section by a distance which is substantially equal to an odd multiple of a quarter waveguide wavelength of said generated harmonic energy propagating in the TEZO mode.

9. The combination of claim 6 wherein said first section of rectangular waveguide is electrically coupled in a closed waveguide loop which is an integral number of waveguide wavelengths long at the frequency of said source, and wherein said source is coupled to said waveguide loop, said second and third waveguide paths being coupled to said closed waveguide loop by directional coupling means which are responsive only to said generated harmonic energy.

Proc. I.R.E., May 1957, vol. 45, pp. 643-646, Microwave Frequency Doubling in Ferrites, Melchor et al. 

